Two main errors can occur in analyzing the action, interaction and correlation of genetic and environmental effects on any trait in human populations. A false hypothesis may be accepted. Effects may be missed which produce substantial bias in other parameter estimates. A detailed computer analysis of simulated data is to be conducted. The resolving power of human kinship studies will be investigated in four main areas: mechanisms of environmental transmission; mechanisms of mate selection; genotype-environment interaction; the interaction of genetic and environmental effects with sex. A number of new and old designs will be examined. Unless these effects can be quantified genetic studies may be seriously misleading. The importance for health-related goals lies in the need to justify any design in terms of its analytical power and cost-effectiveness for resolving competing hypotheses about the action and interaction of genes and environment. The project will determine sample sizes and structure necessary for reliable inference and reveal areas where ambiguities are likely to remain.